It is well known in the optics field to control electromagnetic beams, typically light beams. It is often necessary to sample a portion of a light beam for subsequent control purposes. This typically involves some sort of detector and a feedback loop. Beams are typically detected in terms of their displacement and angle. In the prior art, for instance, to control the beams the lenses present in such a system are sometimes used in conjunction with steering mirrors. Typically, for instance, there are two steering mirrors in the system and two detectors. In one known system it is arranged so that one detector only observes changes in the beam due to the tilt of the first steering mirror. But then it has been found that it is impossible to make the second detector output signal dependent only upon the tilt of the second steering mirror. In other words, this arrangement has undesirable in terms of feedback, making the feedback complicated and almost impossible to eliminate all cross coupling. For example, when a position or angle change occurs to the input light beam which is captured by a non-zero reading from the first detector with a reading of the second detector remaining unchanged, the first steering mirror will have to be moved to eliminate the non-zero reading. This leads to an angle change in the output beam which will be detected in the second detector leading to a correction signal applied to the second mirror. Even if the system is carefully tuned so as to be stable, changes in the relative locations of the steering mirrors and detectors require a complete re-tuning, and may even result in configurations for which no stable tuning is possible. In particular, the arrangement in which one detector only observes changes due to tilt of one steering mirror is only possible at one unique distance between steering mirror and detector, based on the focal length of an interposed lens. This is generally a complicated system and it has undesirably proven almost impossible to eliminate all its error, or to be re-configured in fielded applications which require variation in the optical layout.